1. Field of the Invention
The present invention relates to a vacuum-processing apparatus and method for vacuum-processing an object, including forming a deposited film on a substrate under reduced pressure condition and etching a substrate under reduced pressure condition. Specifically, for instance, the present invention relates to a vacuum-processing apparatus and method which are used in mass-producing a photoelectric conversion element in which a non-single crystal semiconductor material such as an amorphous silicon semiconductor material or the like is used.
2. Related Background Art
There are known a number of elements having a structure comprising a plurality of semiconductor layers composed of a non-single crystal material such as an amorphous material stacked as photoelectric conversion elements which are used in solar cells or the like. In the production of such element, vacuum-processing apparatus capable of forming a thin semiconductor film have been widely used on an industrial scale.
Now, it is basically important for a solar cell that it has a sufficiently high photoelectric conversion efficiency, excels in stability of characteristics, and is able to mass-produce. In view of this, in the production of a solar cell in which a non-single crystal semiconductor layer such as an amorphous semiconductor layer or the like is used, it is necessary that said solar cell is produced so as to have good electric, optical and photoconductive characteristics, physical properties, fatigue resistance upon repeated use, and durability to use environments. And in order to mass-produce such solar cell, it is necessary to adopt a method which makes it possible to repeatedly form a large area semiconductor layer having a uniform thickness and a homogenous property at a high speed to form a photoelectric conversion element which constitutes said solar cell, where said photoelectric conversion element can be mass-produced.
Separately, in a solar cell power generation apparatus (a sunlight power generation apparatus) in which solar cells are used, there is frequently adopted a form in which a plurality of unit solar modules are electrically connected in series connection or parallel connection into an integrated body which outputs a desired electric current and a desired voltage. In this case, it is required that neither disconnection nor short circuit are occurred in each unit solar cell module. In addition, it is important to make such that the solar cell modules are not varied with respect their output electric currents and output voltages. In this connection, at least at a stage of preparing said plurality of unit solar cell modules, it is important that a semiconductor layer constituting each unit solar cell module which is an utmost decisive factor to determine the characteristic of said solar cell module is formed to have an uniform property. By forming a semiconductor having an uniform property for each unit solar cell module in this way, to work out a design for each unit solar cell module can be facilitated and a process of assembling a plurality of unit solar cell modules can be simplified. This situation makes it possible to mass-produce a solar cell power generation apparatus at a reasonable production cost. Thus, there is a demand for provide a film-forming method which enables one to continuously form a semiconductor layer having a uniform property over a large area.
Incidentally, in a solar cell used in a solar cell module, a semiconductor layer which is an important constituent component of said solar cell has a semiconductor junction such as a p-n junction or a p-i-n junction. In the case where the semiconductor layer is constituted by an amorphous silicon (a-Si) semiconductor material, it is known that such semiconductor layer can be formed by a plasma CVD method in that a step of generating plasma discharge in the presence of a raw material gas comprising silane gas (SiH4) which, if necessary, contains a doping gas comprising, for instance, phosphine (PH3) or diborane (B2H6) to decompose said raw material gas whereby forming a semiconductor layer having a desired conduction type on a desired substrate maintained at a desired temperature is repeated to sequentially form a plurality of semiconductor layers having a desired conduction type on said substrate whereby forming a layered semiconductor layer having a desired semiconductor junction on said substrate.
In order to industrially produce a solar cell which can be obtained by stacking a plurality of such amorphous semiconductor layers having a desired conduction type in such manner as above described, there is known a method using a film-forming apparatus having a plurality of independent deposition chambers communicated with each other, in that an elongated substrate is continuously moved to sequentially pass through said plurality of deposition chambers while forming a semiconductor layer having a desired conduction type on said substrate by each deposition chamber to continuously form a stacked body comprising a plurality of semiconductor layers having a desired conduction type and which has a desired semiconductor junction on said substrate.
For instance, U.S. Pat. No. 4,400,409 discloses a roll-to-roll type continuous plasma CVD apparatus capable of continuously forming such stacked body. Particularly, the plasma CVD apparatus disclosed in this document comprises a plurality of plasma discharge deposition chambers capable of being evacuated each for forming a desired semiconductor layer by causing plasma discharge therein, said plurality of plasma discharge deposition chambers being communicated with each other, wherein an elongated substrate (a web substrate) having flexibility and having a desired width is moved in the longitudinal direction and along a prescribed pathway for the substrate to be moved to sequentially pass through the plasma discharge deposition chambers while forming a desired semiconductor layer on the substrate by each plasma discharge deposition chamber, whereby an semiconductor element having a desired semiconductor junction is continuously formed on the substrate. In this plasma CVD apparatus, in order to prevent a raw material gas containing a dopant which is used in one of the plasma discharge deposition chambers from being diffused or contaminated into the other plasma discharge deposition chamber which is situated next to said plasma discharge deposition chamber, a gas gate is provided between each adjacent plasma discharge deposition chambers. Particularly, there is adopted a manner in that a gas gate is established between each adjacent plasma discharge deposition chambers by providing an isolation passageway in a slit-like form between the two discharge deposition chambers and flowing a scavenging gas such as Ar gas or H2 gas into said isolation passageway, and by said gas gate, a raw material gas introduced in one plasma discharge deposition chamber is prevented from being invaded into the other plasma discharge deposition chamber situated next to said plasma discharge deposition chamber.
However, in the case of forming a semiconductor layer in accordance with such semiconductor layer-forming method using the above described roll-to-roll plasma CVD apparatus, there are such disadvantages as will be described below.
(1) When a plasma is continuously generated over a large area in the processing chamber (the plasma discharge deposition chamber) over a long period of time in order to continuously form a semiconductor layer on a substrate therein, there is a tendency of entailing a disadvantage such that the temperature of the circumferential wall of the processing chamber is gradually increased with the passage of time to exert an influence to the temperature of said substrate on which said semiconductor layer is formed, where this situation makes it almost impossible to maintain the substrate at a temperature in a prescribed temperature range. Thus, when a semiconductor device is continuously formed over a long period of time, there is a tendency in that the resulting semiconductor devices have a variation in their characteristics with time.
(2) As described in the above (1), the temperature of the circumferential wall of the processing chamber is increased with the passage of time. Therefore, upon conducting maintenance work of the apparatus after the completion of the layer-forming process, it is necessary to cool not only the processing chamber but also members around the processing chamber until they become to have a temperature which enables a worker to conduct the maintenance work. This situation entails disadvantages such that a long period time is necessitated to be spent in the maintenance work because it takes a long period time until the processing chamber and the members around the processing chamber are cooled as above described and as a result, the operating efficiency of the apparatus is remarkably decreased.
Now, for a solar cell power generation apparatus obtained by electrically connecting a plurality of unit solar cell modules in series connection or parallel connection into an integrated body, in order make the solar cell power generation apparatus have an improved photoelectric conversion efficiency and a improved characteristic stability, it is preferred that each unit solar cell module has a photoelectric conversion efficiency which is as high as possible and a characteristic degradation ratio which is as low as possible. Further, when because the characteristic of the integrated body as a whole is governed by that of one of said plurality of unit solar cell modules which outputs a minimum electric current or a minimum voltage, it is very important to make the respective unit solar cell modules have an improved average characteristic and to make them such that a variation in their characteristics is quite small. For this purpose, at least at a stage of preparing said plurality of unit solar cell modules, it is necessary that a semiconductor layer constituting each unit solar cell module which is an utmost decisive factor to determine the characteristic of said solar cell module is formed to have an uniform property. The foregoing semiconductor layer-forming method using the roll-to-roll type plasma CVD apparatus is not appropriate to sufficiently satisfy this requirement because as described in the above (1), when a semiconductor device is continuously formed over a long period of time, there is a disadvantage in that the resulting semiconductor devices tend to have a variation in their characteristics with time. Further, in order to reduce the production cost of a product, it is necessary to decrease the working period of time required for conducting the maintenance and the like for the apparatus as short as possible whereby increase the operating efficiency of the apparatus. However, this purpose cannot be sufficiently attained in the case of using the foregoing roll-to-roll type plasma CVD apparatus, because as described in the above (2), a long period time is necessitated to be spent in the maintenance work for the apparatus.
Thus, although the foregoing roll-to-roll type plasma CVD apparatus is suitable for mass-producing a semiconductor device having a relatively small area, it is difficult to efficiently mass-produce a semiconductor device having a relatively large area and having a stable characteristic.
The foregoing disadvantages will be more or less entailed in the case of vacuum-processing a substrate over a long period of time.